Most people get to know cryptography when they are asked for their SSH key to gain access to accounts on servers (e.g. in Linux or UNIX environments). As there are some differences, which seem to confuse people, I would like to documen the differences between the RSA Public Key formats. RFC 4716 and StackOverflow provides further input on this topic.
OpenSSH
The most common format is the OpenSSH format. This format is also used in the .ssh/authorized_keys file in most environments:
ssh-rsa AAAAB3NzaC1yc2EAAAABJQAAAQB/nAmOjTmezNUDKYvEeIRf2YnwM9/uUG1d0BYsc8/tRtx+RGi7N2lUbp728MXGwdnL9od4cItzky/zVdLZE2cycOa18xBK9cOWmcKS0A8FYBxEQWJ/q9YVUgZbFKfYGaGQxsER+A0w/fX8ALuk78ktP31K69LcQgxIsl7rNzxsoOQKJ/CIxOGMMxczYTiEoLvQhapFQMs3FL96didKr/QbrfB1WT6s3838SEaXfgZvLef1YB2xmfhbT9OXFE3FXvh2UPBfN+ffE7iiayQf/2XR+8j4N4bW30DiPtOQLGUrH1y5X/rpNZNlWW2+jGIxqZtgWg7lTy3mXy5x836Sj/6L
Secure Shell
Another format, which is sometimes used while generating keys is the Secure Shell (SSH) Public Key file format:
---- BEGIN SSH2 PUBLIC KEY ---- AAAAB3NzaC1yc2EAAAABJQAAAQB/nAmOjTmezNUDKYvEeIRf2YnwM9/uUG1d0BYs c8/tRtx+RGi7N2lUbp728MXGwdnL9od4cItzky/zVdLZE2cycOa18xBK9cOWmcKS 0A8FYBxEQWJ/q9YVUgZbFKfYGaGQxsER+A0w/fX8ALuk78ktP31K69LcQgxIsl7r NzxsoOQKJ/CIxOGMMxczYTiEoLvQhapFQMs3FL96didKr/QbrfB1WT6s3838SEaX fgZvLef1YB2xmfhbT9OXFE3FXvh2UPBfN+ffE7iiayQf/2XR+8j4N4bW30DiPtOQ LGUrH1y5X/rpNZNlWW2+jGIxqZtgWg7lTy3mXy5x836Sj/6L ---- END SSH2 PUBLIC KEY ----
RSA Public Key
Please note the difference of this format to the SSH2 format:
- five –
- no spaces
-----BEGIN RSA PUBLIC KEY----- MIIBCgKCAQEA+xGZ/wcz9ugFpP07Nspo6U17l0YhFiFpxxU4pTk3Lifz9R3zsIsu ERwta7+fWIfxOo208ett/jhskiVodSEt3QBGh4XBipyWopKwZ93HHaDVZAALi/2A +xTBtWdEo7XGUujKDvC2/aZKukfjpOiUI8AhLAfjmlcD/UZ1QPh0mHsglRNCmpCw mwSXA9VNmhz+PiB+Dml4WWnKW/VHo2ujTXxq7+efMU4H2fny3Se3KYOsFPFGZ1TN QSYlFuShWrHPtiLmUdPoP6CV2mML1tk+l7DIIqXrQhLUKDACeM5roMx0kLhUWB8P +0uj1CNlNN4JRZlC7xFfqiMbFRU9Z4N6YwIDAQAB -----END RSA PUBLIC KEY-----
Base-64 encoded
The following base-64 encoded key data is displayed via its hex dump:
00 00 00 07 73 73 68 2d 72 73 61 00 00 00 01 25 00 00 01 00 7f 9c 09 8e 8d 39 9e cc d5 03 29 8b c4 78 84 5f d9 89 f0 33 df ee 50 6d 5d d0 16 2c 73 cf ed 46 dc 7e 44 68 bb 37 69 54 6e 9e f6 f0 c5 c6 c1 d9 cb f6 87 78 70 8b 73 93 2f f3 55 d2 d9 13 67 32 70 e6 b5 f3 10 4a f5 c3 96 99 c2 92 d0 0f 05 60 1c 44 41 62 7f ab d6 15 52 06 5b 14 a7 d8 19 a1 90 c6 c1 11 f8 0d 30 fd f5 fc 00 bb a4 ef c9 2d 3f 7d 4a eb d2 dc 42 0c 48 b2 5e eb 37 3c 6c a0 e4 0a 27 f0 88 c4 e1 8c 33 17 33 61 38 84 a0 bb d0 85 aa 45 40 cb 37 14 bf 7a 76 27 4a af f4 1b ad f0 75 59 3e ac df cd fc 48 46 97 7e 06 6f 2d e7 f5 60 1d b1 99 f8 5b 4f d3 97 14 4d c5 5e f8 76 50 f0 5f 37 e7 df 13 b8 a2 6b 24 1f ff 65 d1 fb c8 f8 37 86 d6 df 40 e2 3e d3 90 2c 65 2b 1f 5c b9 5f fa e9 35 93 65 59 6d be 8c 62 31 a9 9b 60 5a 0e e5 4f 2d e6 5f 2e 71 f3 7e 92 8f fe 8b
Header Encoding
You might notice, that most keys begin with a small prefix: AAAAB3NzaC1yc2E. Thats due to the the public key blob, which consists of a header that indicates, that you have a SSH public key. The prefix encodes this header. RFC 4254 defines the encoding of these public key headers in SSH key format as followed:
string "ssh-rsa"
mpint e # signature key blob
mpint n # signature key blob
The string is encoded as 32-bit big endian length followed by the literal string:
0,0,0,7,'s','s','h','-','r','s','a'
If you encode this sequence of bytes in Base-64, you receive the mentioned header prefix:
AAAAB3NzaC1yc2E
You will notice that also the public exponent is also usually the same, which make the common prefix appear even longer. However, the bulk of the key is the public modulus, which is completely different within different keys.
