On Mon, 22 Jan 2024, Jakub Jelen wrote:
> Hi,
> looking through the key specification, you can see that its the second
> field in the key file:
>
> https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.key#L11
>
> It looks like there is no convenient way to get this information with
> openssh cli, but given that the file format is just base64 encoded,
> you can read it out with something like this:
>
> $ cat /tmp/rsa | head -n -1 | tail -n +2 | base64 -d | hexdump -C | head -n 2
> 00000000 6f 70 65 6e 73 73 68 2d 6b 65 79 2d 76 31 00 00 |openssh-key-v1..|
> 00000010 00 00 0a 61 65 73 32 35 36 2d 63 74 72 00 00 00 |...aes256-ctr...|
>
> In this case, this key is aes256-ctr encrypted.
You could use something like the attached python script if you don't want
to stare at hexdumps :)
#!/usr/bin/env python3
import sys
import base64
BEGIN="-----BEGIN OPENSSH PRIVATE KEY-----\n"
END="\n-----END OPENSSH PRIVATE KEY-----"
BLOBSTART=b"openssh-key-v1\x00"
for f in sys.argv[1:]:
d64 = open(f, "rt").read()
o = d64.find(BEGIN)
if o == -1:
raise ValueError("{} missing begin marker".format(f))
d64 = d64[o + len(BEGIN):]
o = d64.find(END)
if o == -1:
raise ValueError("{} missing end marker".format(f))
d64 = d64[:o]
d = base64.b64decode(d64)
if d[:len(BLOBSTART)] != BLOBSTART:
raise ValueError("{} missing blob preamble".format(f))
d = d[len(BLOBSTART):]
l = (d[0] << 24) | (d[1] << 16) | (d[2] << 8) | d[3]
alg = d[4: 4+l].decode("utf-8")
print("{}: {}".format(f, alg))
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