ssh-agent libssh2_agent_userauth error, works on 1.10.0

Describe the bug
Hi there,

We're excited about the rsa-sha2-256/512 support in libssh2, however we encountered a bug with ssh-agent support reproduced with the examples.

Everything works fine when using keys from disk, however when trying to auth to a server using libssh2_agent_userauth we get LIBSSH2_ERROR_PUBLICKEY_UNVERIFIED. The same exact code works with 1.10.0 (when accessing a server that accepts ssh-rsa).

See below for output from a few different scenarios, along with a dockerfile and patch used for testing.

Potentially related: #536 and #626

To Reproduce
./example/example-ssh2 <server-ip> git: success
./example/example-ssh2_agent <server-ip> git: error

See test patch below used to provide output, but reproducible without any patches.

Expected behavior
libssh2_agent_userauth should auth successfully.

Version (please complete the following information):

OS: Fedora 35
libssh2 version: master (13ad7b2)

Additional context
1.10.0 example-ssh2 PubkeyAcceptedKeyTypes=ssh-ed25519,rsa-sha2-256,ssh-rsa (Success expected):

Connecting to git@172.17.0.2. pass: password
[libssh2] 0.572899 Key Ex: Sent KEX: curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1,diffie-hellman-group-exchange-sha1
[libssh2] 0.572909 Key Ex: Sent HOSTKEY: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519,ssh-rsa,ssh-dss
[libssh2] 0.572913 Key Ex: Sent CRYPT_CS: aes128-ctr,aes192-ctr,aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-cbc,aes128-cbc,blowfish-cbc,arcfour128,arcfour,cast128-cbc,3des-cbc
[libssh2] 0.572915 Key Ex: Sent CRYPT_SC: aes128-ctr,aes192-ctr,aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-cbc,aes128-cbc,blowfish-cbc,arcfour128,arcfour,cast128-cbc,3des-cbc
[libssh2] 0.572916 Key Ex: Sent MAC_CS: hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-sha1-96,hmac-md5,hmac-md5-96,hmac-ripemd160,hmac-ripemd160@openssh.com
[libssh2] 0.572918 Key Ex: Sent MAC_SC: hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-sha1-96,hmac-md5,hmac-md5-96,hmac-ripemd160,hmac-ripemd160@openssh.com
[libssh2] 0.572919 Key Ex: Sent COMP_CS: none
[libssh2] 0.572921 Key Ex: Sent COMP_SC: none
[libssh2] 0.572922 Key Ex: Sent LANG_CS: 
[libssh2] 0.572924 Key Ex: Sent LANG_SC: 
[libssh2] 0.573698 Key Ex: Agreed on KEX method: curve25519-sha256
[libssh2] 0.573704 Key Ex: Agreed on HOSTKEY method: ecdsa-sha2-nistp256
[libssh2] 0.573705 Key Ex: Agreed on CRYPT_CS method: aes128-ctr
[libssh2] 0.573706 Key Ex: Agreed on CRYPT_SC method: aes128-ctr
[libssh2] 0.573707 Key Ex: Agreed on MAC_CS method: hmac-sha2-256
[libssh2] 0.573708 Key Ex: Agreed on MAC_SC method: hmac-sha2-256
[libssh2] 0.573708 Key Ex: Agreed on COMP_CS method: none
[libssh2] 0.573709 Key Ex: Agreed on COMP_SC method: none
[libssh2] 0.573778 Key Ex: Initiating curve25519 SHA2
[libssh2] 0.577639 Key Ex: Server's MD5 Fingerprint: af:61:b0:ad:fa:6e:3b:b6:c4:0b:4a:ee:34:85:9a:f1
[libssh2] 0.577651 Key Ex: Server's SHA1 Fingerprint: 3d:eb:f3:d1:63:0b:22:5e:e6:a8:46:f0:61:88:f7:c0:50:6a:11:5c
[libssh2] 0.577655 Key Ex: Server's SHA256 Fingerprint: fDDUXe6r2q+TCFIhzZdck5R2zj1TaMiM/JL7gLsUhoA=
[libssh2] 0.577898 Key Ex: Received NEWKEYS message
[libssh2] 0.577903 Key Ex: session_id calculated
[libssh2] 0.577909 Key Ex: Client to Server IV and Key calculated
[libssh2] 0.577913 Key Ex: Server to Client IV and Key calculated
[libssh2] 0.577915 Key Ex: Client to Server HMAC Key calculated
[libssh2] 0.577917 Key Ex: Server to Client HMAC Key calculated
[libssh2] 0.577918 Key Ex: Client to Server compression initialized
[libssh2] 0.577918 Key Ex: Server to Client compression initialized
Fingerprint: 3D EB F3 D1 63 0B 22 5E E6 A8 46 F0 61 88 F7 C0 50 6A 11 5C 
[libssh2] 0.585770 Userauth: Permitted auth methods: publickey
Authentication methods: publickey
[libssh2] 0.585784 Userauth: Loading public key file: /home/ian/.ssh/pubs/id_rsa_test.pub
[libssh2] 0.585830 Userauth: Attempting publickey authentication
[libssh2] 0.586355 Userauth: Loading private key file: /home/ian/.ssh/id_rsa_test
[libssh2] 0.586436 Userauth: Computing RSA keys from private key data
[libssh2] 0.588955 Userauth: Attempting publickey authentication -- phase 2
[libssh2] 0.601704 Userauth: Publickey authentication successful
        Authentication by public key succeeded.
all done!

1.10.0 example-ssh2 PubkeyAcceptedKeyTypes=ssh-ed25519,rsa-sha2-256 (Failure expected):

Connecting to git@172.17.0.2. pass: password
[libssh2] 0.507683 Key Ex: Sent KEX: curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1,diffie-hellman-group-exchange-sha1
[libssh2] 0.507699 Key Ex: Sent HOSTKEY: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519,ssh-rsa,ssh-dss
[libssh2] 0.507701 Key Ex: Sent CRYPT_CS: aes128-ctr,aes192-ctr,aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-cbc,aes128-cbc,blowfish-cbc,arcfour128,arcfour,cast128-cbc,3des-cbc
[libssh2] 0.507703 Key Ex: Sent CRYPT_SC: aes128-ctr,aes192-ctr,aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-cbc,aes128-cbc,blowfish-cbc,arcfour128,arcfour,cast128-cbc,3des-cbc
[libssh2] 0.507704 Key Ex: Sent MAC_CS: hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-sha1-96,hmac-md5,hmac-md5-96,hmac-ripemd160,hmac-ripemd160@openssh.com
[libssh2] 0.507705 Key Ex: Sent MAC_SC: hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-sha1-96,hmac-md5,hmac-md5-96,hmac-ripemd160,hmac-ripemd160@openssh.com
[libssh2] 0.507707 Key Ex: Sent COMP_CS: none
[libssh2] 0.507708 Key Ex: Sent COMP_SC: none
[libssh2] 0.507708 Key Ex: Sent LANG_CS: 
[libssh2] 0.507709 Key Ex: Sent LANG_SC: 
[libssh2] 0.508503 Key Ex: Agreed on KEX method: curve25519-sha256
[libssh2] 0.508510 Key Ex: Agreed on HOSTKEY method: ecdsa-sha2-nistp256
[libssh2] 0.508511 Key Ex: Agreed on CRYPT_CS method: aes128-ctr
[libssh2] 0.508512 Key Ex: Agreed on CRYPT_SC method: aes128-ctr
[libssh2] 0.508513 Key Ex: Agreed on MAC_CS method: hmac-sha2-256
[libssh2] 0.508514 Key Ex: Agreed on MAC_SC method: hmac-sha2-256
[libssh2] 0.508515 Key Ex: Agreed on COMP_CS method: none
[libssh2] 0.508516 Key Ex: Agreed on COMP_SC method: none
[libssh2] 0.508600 Key Ex: Initiating curve25519 SHA2
[libssh2] 0.512767 Key Ex: Server's MD5 Fingerprint: af:61:b0:ad:fa:6e:3b:b6:c4:0b:4a:ee:34:85:9a:f1
[libssh2] 0.512779 Key Ex: Server's SHA1 Fingerprint: 3d:eb:f3:d1:63:0b:22:5e:e6:a8:46:f0:61:88:f7:c0:50:6a:11:5c
[libssh2] 0.512783 Key Ex: Server's SHA256 Fingerprint: fDDUXe6r2q+TCFIhzZdck5R2zj1TaMiM/JL7gLsUhoA=
[libssh2] 0.513041 Key Ex: Received NEWKEYS message
[libssh2] 0.513046 Key Ex: session_id calculated
[libssh2] 0.513055 Key Ex: Client to Server IV and Key calculated
[libssh2] 0.513059 Key Ex: Server to Client IV and Key calculated
[libssh2] 0.513063 Key Ex: Client to Server HMAC Key calculated
[libssh2] 0.513065 Key Ex: Server to Client HMAC Key calculated
[libssh2] 0.513067 Key Ex: Client to Server compression initialized
[libssh2] 0.513069 Key Ex: Server to Client compression initialized
Fingerprint: 3D EB F3 D1 63 0B 22 5E E6 A8 46 F0 61 88 F7 C0 50 6A 11 5C 
[libssh2] 0.518451 Userauth: Permitted auth methods: publickey
Authentication methods: publickey
[libssh2] 0.518460 Userauth: Loading public key file: /home/ian/.ssh/pubs/id_rsa_test.pub
[libssh2] 0.518494 Userauth: Attempting publickey authentication
        Authentication by public key failed!
        Error (-18): Username/PublicKey combination invalid
all done!

1.10.0 example-ssh2_agent PubkeyAcceptedKeyTypes=ssh-ed25519,rsa-sha2-256,ssh-rsa (Success expected):

[libssh2] 0.118770 Key Ex: Sent KEX: curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1,diffie-hellman-group-exchange-sha1
[libssh2] 0.118786 Key Ex: Sent HOSTKEY: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519,ssh-rsa,ssh-dss
[libssh2] 0.118788 Key Ex: Sent CRYPT_CS: aes128-ctr,aes192-ctr,aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-cbc,aes128-cbc,blowfish-cbc,arcfour128,arcfour,cast128-cbc,3des-cbc
[libssh2] 0.118789 Key Ex: Sent CRYPT_SC: aes128-ctr,aes192-ctr,aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-cbc,aes128-cbc,blowfish-cbc,arcfour128,arcfour,cast128-cbc,3des-cbc
[libssh2] 0.118790 Key Ex: Sent MAC_CS: hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-sha1-96,hmac-md5,hmac-md5-96,hmac-ripemd160,hmac-ripemd160@openssh.com
[libssh2] 0.118791 Key Ex: Sent MAC_SC: hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-sha1-96,hmac-md5,hmac-md5-96,hmac-ripemd160,hmac-ripemd160@openssh.com
[libssh2] 0.118792 Key Ex: Sent COMP_CS: none
[libssh2] 0.118793 Key Ex: Sent COMP_SC: none
[libssh2] 0.118794 Key Ex: Sent LANG_CS: 
[libssh2] 0.118795 Key Ex: Sent LANG_SC: 
[libssh2] 0.119483 Key Ex: Agreed on KEX method: curve25519-sha256
[libssh2] 0.119489 Key Ex: Agreed on HOSTKEY method: ecdsa-sha2-nistp256
[libssh2] 0.119490 Key Ex: Agreed on CRYPT_CS method: aes128-ctr
[libssh2] 0.119491 Key Ex: Agreed on CRYPT_SC method: aes128-ctr
[libssh2] 0.119492 Key Ex: Agreed on MAC_CS method: hmac-sha2-256
[libssh2] 0.119493 Key Ex: Agreed on MAC_SC method: hmac-sha2-256
[libssh2] 0.119494 Key Ex: Agreed on COMP_CS method: none
[libssh2] 0.119495 Key Ex: Agreed on COMP_SC method: none
[libssh2] 0.119563 Key Ex: Initiating curve25519 SHA2
[libssh2] 0.123372 Key Ex: Server's MD5 Fingerprint: af:61:b0:ad:fa:6e:3b:b6:c4:0b:4a:ee:34:85:9a:f1
[libssh2] 0.123386 Key Ex: Server's SHA1 Fingerprint: 3d:eb:f3:d1:63:0b:22:5e:e6:a8:46:f0:61:88:f7:c0:50:6a:11:5c
[libssh2] 0.123391 Key Ex: Server's SHA256 Fingerprint: fDDUXe6r2q+TCFIhzZdck5R2zj1TaMiM/JL7gLsUhoA=
[libssh2] 0.123632 Key Ex: Received NEWKEYS message
[libssh2] 0.123636 Key Ex: session_id calculated
[libssh2] 0.123643 Key Ex: Client to Server IV and Key calculated
[libssh2] 0.123646 Key Ex: Server to Client IV and Key calculated
[libssh2] 0.123648 Key Ex: Client to Server HMAC Key calculated
[libssh2] 0.123650 Key Ex: Server to Client HMAC Key calculated
[libssh2] 0.123651 Key Ex: Client to Server compression initialized
[libssh2] 0.123652 Key Ex: Server to Client compression initialized
Fingerprint: 3D EB F3 D1 63 0B 22 5E E6 A8 46 F0 61 88 F7 C0 50 6A 11 5C 
[libssh2] 0.131866 Userauth: Permitted auth methods: publickey
Authentication methods: publickey
[libssh2] 0.132304 Userauth: Attempting publickey authentication
[libssh2] 0.136196 Userauth: Attempting publickey authentication -- phase 2
[libssh2] 0.148798 Userauth: Publickey authentication successful
        Authentication with username git and public key ian@ianh-dell succeeded!
all done!

master (13ad7b2) example-ssh2 PubkeyAcceptedKeyTypes=ssh-ed25519,rsa-sha2-512,rsa-sha2-256 (Success expected):

Connecting to git@172.17.0.2. pass: password
[libssh2] 0.320826 Key Ex: Sent KEX: curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1,diffie-hellman-group-exchange-sha1,ext-info-c
[libssh2] 0.320846 Key Ex: Sent HOSTKEY: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa,ssh-dss
[libssh2] 0.320849 Key Ex: Sent CRYPT_CS: aes128-ctr,aes192-ctr,aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-cbc,aes128-cbc,blowfish-cbc,arcfour128,arcfour,cast128-cbc,3des-cbc
[libssh2] 0.320850 Key Ex: Sent CRYPT_SC: aes128-ctr,aes192-ctr,aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-cbc,aes128-cbc,blowfish-cbc,arcfour128,arcfour,cast128-cbc,3des-cbc
[libssh2] 0.320852 Key Ex: Sent MAC_CS: hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-sha1-96,hmac-md5,hmac-md5-96,hmac-ripemd160,hmac-ripemd160@openssh.com
[libssh2] 0.320854 Key Ex: Sent MAC_SC: hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-sha1-96,hmac-md5,hmac-md5-96,hmac-ripemd160,hmac-ripemd160@openssh.com
[libssh2] 0.320855 Key Ex: Sent COMP_CS: none
[libssh2] 0.320856 Key Ex: Sent COMP_SC: none
[libssh2] 0.320858 Key Ex: Sent LANG_CS: 
[libssh2] 0.320859 Key Ex: Sent LANG_SC: 
[libssh2] 0.321839 Key Ex: Agreed on KEX method: curve25519-sha256
[libssh2] 0.321849 Key Ex: Agreed on HOSTKEY method: ecdsa-sha2-nistp256
[libssh2] 0.321852 Key Ex: Agreed on CRYPT_CS method: aes128-ctr
[libssh2] 0.321853 Key Ex: Agreed on CRYPT_SC method: aes128-ctr
[libssh2] 0.321854 Key Ex: Agreed on MAC_CS method: hmac-sha2-256
[libssh2] 0.321857 Key Ex: Agreed on MAC_SC method: hmac-sha2-256
[libssh2] 0.321859 Key Ex: Agreed on COMP_CS method: none
[libssh2] 0.321861 Key Ex: Agreed on COMP_SC method: none
[libssh2] 0.321956 Key Ex: Initiating curve25519 SHA2
[libssh2] 0.327217 Key Ex: Server's MD5 Fingerprint: af:61:b0:ad:fa:6e:3b:b6:c4:0b:4a:ee:34:85:9a:f1
[libssh2] 0.327236 Key Ex: Server's SHA1 Fingerprint: 3d:eb:f3:d1:63:0b:22:5e:e6:a8:46:f0:61:88:f7:c0:50:6a:11:5c
[libssh2] 0.327246 Key Ex: Server's SHA256 Fingerprint: fDDUXe6r2q+TCFIhzZdck5R2zj1TaMiM/JL7gLsUhoA=
[libssh2] 0.327633 Key Ex: Received NEWKEYS message
[libssh2] 0.327641 Key Ex: session_id calculated
[libssh2] 0.327652 Key Ex: Client to Server IV and Key calculated
[libssh2] 0.327657 Key Ex: Server to Client IV and Key calculated
[libssh2] 0.327660 Key Ex: Client to Server HMAC Key calculated
[libssh2] 0.327663 Key Ex: Server to Client HMAC Key calculated
[libssh2] 0.327664 Key Ex: Client to Server compression initialized
[libssh2] 0.327665 Key Ex: Server to Client compression initialized
[libssh2] 0.327711 Key Ex: Server to Client extension server-sig-algs: ssh-ed25519,sk-ssh-ed25519@openssh.com,ssh-rsa,rsa-sha2-256,rsa-sha2-512,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,sk-ecdsa-sha2-nistp256@openssh.com,webauthn-sk-ecdsa-sha2-nistp256@openssh.com
Fingerprint: 3D EB F3 D1 63 0B 22 5E E6 A8 46 F0 61 88 F7 C0 50 6A 11 5C 
[libssh2] 0.333604 Userauth: Permitted auth methods: publickey
Authentication methods: publickey
[libssh2] 0.333623 Userauth: Loading public key file: /home/ian/.ssh/pubs/id_rsa_test.pub
[libssh2] 0.333687 Key Ex: Signing using rsa-sha2-512
[libssh2] 0.333692 Userauth: Attempting publickey authentication
[libssh2] 0.334412 Userauth: Loading private key file: /home/ian/.ssh/id_rsa_test
[libssh2] 0.334519 Userauth: Computing RSA keys from private key data
[libssh2] 0.338672 Userauth: Attempting publickey authentication -- phase 2
[libssh2] 0.345583 Userauth: Publickey authentication successful
        Authentication by public key succeeded.
all done!

master (13ad7b2) example-ssh2_agent PubkeyAcceptedKeyTypes=ssh-ed25519,rsa-sha2-512,rsa-sha2-256[,ssh-rsa] (Failure NOT expected):

[libssh2] 0.020891 Key Ex: Sent KEX: curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1,diffie-hellman-group-exchange-sha1,ext-info-c
[libssh2] 0.020911 Key Ex: Sent HOSTKEY: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa,ssh-dss
[libssh2] 0.020913 Key Ex: Sent CRYPT_CS: aes128-ctr,aes192-ctr,aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-cbc,aes128-cbc,blowfish-cbc,arcfour128,arcfour,cast128-cbc,3des-cbc
[libssh2] 0.020914 Key Ex: Sent CRYPT_SC: aes128-ctr,aes192-ctr,aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-cbc,aes128-cbc,blowfish-cbc,arcfour128,arcfour,cast128-cbc,3des-cbc
[libssh2] 0.020916 Key Ex: Sent MAC_CS: hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-sha1-96,hmac-md5,hmac-md5-96,hmac-ripemd160,hmac-ripemd160@openssh.com
[libssh2] 0.020917 Key Ex: Sent MAC_SC: hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-sha1-96,hmac-md5,hmac-md5-96,hmac-ripemd160,hmac-ripemd160@openssh.com
[libssh2] 0.020918 Key Ex: Sent COMP_CS: none
[libssh2] 0.020919 Key Ex: Sent COMP_SC: none
[libssh2] 0.020920 Key Ex: Sent LANG_CS: 
[libssh2] 0.020921 Key Ex: Sent LANG_SC: 
[libssh2] 0.021780 Key Ex: Agreed on KEX method: curve25519-sha256
[libssh2] 0.021788 Key Ex: Agreed on HOSTKEY method: ecdsa-sha2-nistp256
[libssh2] 0.021790 Key Ex: Agreed on CRYPT_CS method: aes128-ctr
[libssh2] 0.021791 Key Ex: Agreed on CRYPT_SC method: aes128-ctr
[libssh2] 0.021791 Key Ex: Agreed on MAC_CS method: hmac-sha2-256
[libssh2] 0.021793 Key Ex: Agreed on MAC_SC method: hmac-sha2-256
[libssh2] 0.021793 Key Ex: Agreed on COMP_CS method: none
[libssh2] 0.021794 Key Ex: Agreed on COMP_SC method: none
[libssh2] 0.021867 Key Ex: Initiating curve25519 SHA2
[libssh2] 0.025822 Key Ex: Server's MD5 Fingerprint: af:61:b0:ad:fa:6e:3b:b6:c4:0b:4a:ee:34:85:9a:f1
[libssh2] 0.025833 Key Ex: Server's SHA1 Fingerprint: 3d:eb:f3:d1:63:0b:22:5e:e6:a8:46:f0:61:88:f7:c0:50:6a:11:5c
[libssh2] 0.025838 Key Ex: Server's SHA256 Fingerprint: fDDUXe6r2q+TCFIhzZdck5R2zj1TaMiM/JL7gLsUhoA=
[libssh2] 0.026104 Key Ex: Received NEWKEYS message
[libssh2] 0.026109 Key Ex: session_id calculated
[libssh2] 0.026116 Key Ex: Client to Server IV and Key calculated
[libssh2] 0.026120 Key Ex: Server to Client IV and Key calculated
[libssh2] 0.026122 Key Ex: Client to Server HMAC Key calculated
[libssh2] 0.026124 Key Ex: Server to Client HMAC Key calculated
[libssh2] 0.026125 Key Ex: Client to Server compression initialized
[libssh2] 0.026126 Key Ex: Server to Client compression initialized
[libssh2] 0.026158 Key Ex: Server to Client extension server-sig-algs: ssh-ed25519,sk-ssh-ed25519@openssh.com,ssh-rsa,rsa-sha2-256,rsa-sha2-512,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,sk-ecdsa-sha2-nistp256@openssh.com,webauthn-sk-ecdsa-sha2-nistp256@openssh.com
Fingerprint: 3D EB F3 D1 63 0B 22 5E E6 A8 46 F0 61 88 F7 C0 50 6A 11 5C 
[libssh2] 0.032922 Userauth: Permitted auth methods: publickey
Authentication methods: publickey
[libssh2] 0.033701 Key Ex: Signing using rsa-sha2-512
[libssh2] 0.033712 Userauth: Attempting publickey authentication
[libssh2] 0.037304 Userauth: Attempting publickey authentication -- phase 2
        Authentication with username git and public key ian@ianh-dell failed!
        Error (-19): Invalid signature for supplied public key, or bad username/public key combination
Couldn't continue authentication
all done!

Dockerfile

FROM debian:stable

RUN apt update && apt -y install git openssh-server \
  && adduser --disabled-password --gecos "" git \
  && runuser -l git -c 'mkdir .ssh && chmod 700 .ssh && touch .ssh/authorized_keys && chmod 600 .ssh/authorized_keys' \
  && mkdir -p /srv/git/project.git && chown -R git:git /srv/git \
  && runuser -l git -c 'cd /srv/git/project.git && git init --bare' \
  && echo "PubkeyAcceptedKeyTypes=ssh-ed25519,rsa-sha2-512,rsa-sha2-256\nPasswordAuthentication no" >> /etc/ssh/sshd_config

COPY --chown=git:git authorized_keys /home/git/.ssh/authorized_keys

ENTRYPOINT service ssh restart && bash

# On host machine:
# Ensure authorized_keys file is in same directory as sshd.Dockerfile
# sudo docker build -t sshd-git -f sshd.Dockerfile . # build image
# sudo docker run --name sshd-git_1 --rm -it sshd-git # run container, interactive, remove on close
# sudo docker inspect -f '{{range.NetworkSettings.Networks}}{{.IPAddress}}{{end}}' sshd-git_1 # get IP address
# git remote is git@<ip>:/srv/git/project.git

# On container:
# modify accepted protocols (allow old clients to connect):
# sed -i "s/PubkeyAcceptedKeyTypes=.*/PubkeyAcceptedKeyTypes=ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa/g" /etc/ssh/sshd_config && service ssh restart

# NOTE: openssh-server doesn't seem to account for PubkeyAcceptedKeyTypes when sending server-sig-algs
# (after removing rsa-sha2-512 from PubkeyAcceptedKeyTypes, openssh-server still sends it in server-sig-algs. The client attempts to use rsa-sha2-512 and the server rejects it):
# https://lists.archive.carbon60.com/openssh/dev/73729?page=last

Testing patch

diff --git a/example/ssh2.c b/example/ssh2.c
index fa86f55..19a3fc1 100644
--- a/example/ssh2.c
+++ b/example/ssh2.c
@@ -37,8 +37,8 @@
 #include <ctype.h>
 
 
-const char *keyfile1 = "~/.ssh/id_rsa.pub";
-const char *keyfile2 = "~/.ssh/id_rsa";
+const char *keyfile1 = "/home/ian/.ssh/pubs/id_rsa_test.pub";
+const char *keyfile2 = "/home/ian/.ssh/id_rsa_test";
 const char *username = "username";
 const char *password = "password";
 
@@ -98,6 +98,7 @@ int main(int argc, char *argv[])
         password = argv[3];
     }
 
+    printf("Connecting to %s@%s. pass: %s\n", username, argc > 1 ? argv[1] : "localhost", password);
     rc = libssh2_init(0);
     if(rc != 0) {
         fprintf(stderr, "libssh2 initialization failed (%d)\n", rc);
@@ -122,6 +123,7 @@ int main(int argc, char *argv[])
      * banners, exchange keys, and setup crypto, compression, and MAC layers
      */
     session = libssh2_session_init();
+    libssh2_trace(session, LIBSSH2_TRACE_AUTH | LIBSSH2_TRACE_KEX);
     if(libssh2_session_handshake(session, sock)) {
         fprintf(stderr, "Failure establishing SSH session\n");
         return -1;
@@ -190,9 +192,14 @@ int main(int argc, char *argv[])
     }
     else if(auth_pw & 4) {
         /* Or by public key */
-        if(libssh2_userauth_publickey_fromfile(session, username, keyfile1,
-                                               keyfile2, password)) {
+        int rc = libssh2_userauth_publickey_fromfile(session, username, keyfile1,
+                                                     keyfile2, password);
+        if (rc)
+        {
             fprintf(stderr, "\tAuthentication by public key failed!\n");
+            char *msg;
+            libssh2_session_last_error(session, &msg, NULL, 0);
+            fprintf(stderr, "\tError (%d): %s\n", rc, msg);
             goto shutdown;
         }
         else {
diff --git a/example/ssh2_agent.c b/example/ssh2_agent.c
index 1cc5084..8b514ea 100644
--- a/example/ssh2_agent.c
+++ b/example/ssh2_agent.c
@@ -101,6 +101,7 @@ int main(int argc, char *argv[])
      * banners, exchange keys, and setup crypto, compression, and MAC layers
      */
     session = libssh2_session_init();
+    libssh2_trace(session, LIBSSH2_TRACE_AUTH | LIBSSH2_TRACE_KEX);
     if(libssh2_session_handshake(session, sock)) {
         fprintf(stderr, "Failure establishing SSH session\n");
         return 1;
@@ -153,10 +154,14 @@ int main(int argc, char *argv[])
             rc = 1;
             goto shutdown;
         }
-        if(libssh2_agent_userauth(agent, username, identity)) {
+        int rc = libssh2_agent_userauth(agent, username, identity);
+        if(rc) {
             fprintf(stderr, "\tAuthentication with username %s and "
                    "public key %s failed!\n",
                    username, identity->comment);
+            char *msg;
+            libssh2_session_last_error(session, &msg, NULL, 0);
+            fprintf(stderr, "\tError (%d): %s\n", rc, msg);
         }
         else {
             fprintf(stderr, "\tAuthentication with username %s and "

Please let me know if there's anything else I can do to assist with debugging.

Thanks for the report. In the unexpected failure case, what keys are in your agent? I see you're testing with your own keys, what are they? How did you generate them?

Thanks for the quick response. The only key was a fresh RSA key generated with the default options of ssh-keygen:

% ssh-add -l                                                                                                                             130 ↵ ✹ ✭
3072 SHA256:5baJVAQZ+BuKwtwr2By5H95t6ylCHt5NgmflOVL9CUg ian@ianh-dell (RSA)

One more question, is this the line of code you used in your sshd config file in the failure case? PubkeyAcceptedKeyTypes=ssh-ed25519,rsa-sha2-512,rsa-sha2-256[,ssh-rsa]

Sorry, could've been more clear. I meant to indicate that both PubkeyAcceptedKeyTypes=ssh-ed25519,rsa-sha2-512,rsa-sha2-256 and PubkeyAcceptedKeyTypes=ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa fail. And actually, removing the line (leaving the defaults) fails as well.

Investigating this, the agent doesn't support key upgrading and only supports the original ssh-rsa algo. We are not handling the agent algo downgrade so it is failing.

FYI the following patch with a hardcoded flag lets us sign rsa-sha2-512 messages and authenticate successfully.

See:
https://tools.ietf.org/id/draft-miller-ssh-agent-01.html#rfc.section.4.5.1
https://tools.ietf.org/id/draft-miller-ssh-agent-01.html#rfc.section.7.3

diff --git a/src/agent.c b/src/agent.c
index a526c77..a98f676 100644
--- a/src/agent.c
+++ b/src/agent.c
@@ -391,7 +391,7 @@ agent_sign(LIBSSH2_SESSION *session, unsigned char **sig, size_t *sig_len,
         _libssh2_store_str(&s, (const char *)data, data_len);
 
         /* flags */
-        _libssh2_store_u32(&s, 0);
+        _libssh2_store_u32(&s, 4);
 
         transctx->request_len = s - transctx->request;
         transctx->send_recv_total = 0;

Looking to see if LIBSSH2_RSA_SHA2 is defined and then setting it to 6 is probably a good idea. However it doesn't allow downgrading for older agents.

I tried that at first but it looks like even though it's a bit field you can only request 1 algorithm at a time. Setting to 6 fails the signature. Most likely we need to make the decision based on what the server sends us in server-sig-algs.

I think the flag has to match with session->userauth_pblc_method

Beat me to it. :) Compare it to that value and adjust as needed.

I'm still looking into the best approach for downgrading...

Not trivial to pass session->userauth_pblc_method or the flag value to agent_sign function

Maybe create a struct which contais the flag vallue (default 0) and LIBSSH2_AGENT * and use that struct as the abstract param
And before call to sign_callback if session->userauth_pblc_method is sha2-xxx adjust the flag value

How about something like this? I'd like to test the case where SHA2 isn't supported by the server but can't find a config option to stop openssh-server from sending it, even if I remove it via PubkeyAcceptedKeyTypes...

diff --git a/src/agent.c b/src/agent.c
index a526c77..f763c42 100644
--- a/src/agent.c
+++ b/src/agent.c
@@ -94,6 +94,10 @@
 #define SSH_AGENT_CONSTRAIN_LIFETIME 1
 #define SSH_AGENT_CONSTRAIN_CONFIRM 2
 
+/* Signature request methods */
+#define SSH_AGENT_RSA_SHA2_256 2
+#define SSH_AGENT_RSA_SHA2_512 4
+
 #ifdef PF_UNIX
 static int
 agent_connect_unix(LIBSSH2_AGENT *agent)
@@ -375,6 +379,7 @@ agent_sign(LIBSSH2_SESSION *session, unsigned char **sig, size_t *sig_len,
     ssize_t method_len;
     unsigned char *s;
     int rc;
+    uint32_t sign_flags = 0;
 
     /* Create a request to sign the data */
     if(transctx->state == agent_NB_state_init) {
@@ -391,7 +396,14 @@ agent_sign(LIBSSH2_SESSION *session, unsigned char **sig, size_t *sig_len,
         _libssh2_store_str(&s, (const char *)data, data_len);
 
         /* flags */
-        _libssh2_store_u32(&s, 0);
+        if (session->userauth_pblc_method_len > 0 && session->userauth_pblc_method) {
+            if(!strncmp(session->userauth_pblc_method, "rsa-sha2-512", session->userauth_pblc_method_len)) {
+                sign_flags |= SSH_AGENT_RSA_SHA2_512;
+            } else if(!strncmp(session->userauth_pblc_method, "rsa-sha2-256", session->userauth_pblc_method_len)) {
+                sign_flags |= SSH_AGENT_RSA_SHA2_256;
+            }
+        }
+        _libssh2_store_u32(&s, sign_flags);
 
         transctx->request_len = s - transctx->request;
         transctx->send_recv_total = 0;

Ohh we have acess to session, tested the diff and working for me.

You can adjust the supported algos by configuring sshd using pubkeyacceptedalgorithms.

Tried that before in /etc/ssh/sshd_config (pubkeyacceptedalgorithms=ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa, also PubkeyAcceptedAlgorithms) but get a bad configuration option error, this is with openssh-server 8.4p1 on debian

root@821e01f8aba4:/home/git/.ssh# service ssh restart
/etc/ssh/sshd_config: line 125: Bad configuration option: pubkeyacceptedalgorithms
/etc/ssh/sshd_config: terminating, 1 bad configuration options

Also tried

Match User git
        PubkeyAcceptedAlgorithms=ssh-rsa

looks like they may have renamed it at some point, try pubkeyacceptedkeytypes

Yeah that's what I was using before (see the Dockerfile in the description), but the server still sends all the algorithms it supports regardless of this setting. If the client then sends something that was disabled via pubkeyacceptedkeytypes the server will fail the auth. Super annoying. There's even a note about it in the source: https://github.com/openssh/openssh-portable/blob/d9dbb5d9a0326e252d3c7bc13beb9c2434f59409/kex.c#L437

Legacy Agent support for rsa2 key upgrading/downgrading libssh2#659

Legacy Agent support for rsa2 key upgrading/downgrading #659 (#662)

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ssh-agent libssh2_agent_userauth error, works on 1.10.0 #659

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ssh-agent libssh2_agent_userauth error, works on 1.10.0 #659

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willco007 commented on Jan 12, 2022

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