I have a public key generated with ssh-keygen and I'm just wondering how I get information on the keylength with openssl?
asked Sep 11, 2013 at 17:12
-
The quick and dirty: Modulus bits ~= Pub key bytes * 6– MickLHCommented Jun 13, 2017 at 18:47
Add a comment
|
2 Answers
With openssl, if your private key is in the file id_rsa, then
openssl rsa -text -noout -in id_rsa
will print the private key contents, and the first line of output contains the modulus size in bits. If the key is protected by a passphrase you will have to enter that passphrase, of course.
If you only have the public key, then OpenSSL won't help directly. @Enigma shows the proper command line (with ssh-keygen -lf id_rsa.pub). You can still do that with OpenSSL the following way:
Open the public key file with a text editor. You will find something like this:
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDDo2xko99piegEDgZCrobfFTvXUTFDbWT
ch4IGk5mk0CelB5RKiCvDeK4yhDLcj8QNumaReuwNKGjAQwdENsIT1UjOdVvZOX2d41/p6J
gOCD1ujjwuHWBzzQvDA5rXdQgsdsrJIfNuYr/+kIIANkGPPIheb2Ar2ccIWh9giwNHDjkXT
JXTVQ5Whc0mGBU/EGdlCD6poG4EzCc0N9zk/DNSMIIZUInySaHhn2f7kmfoh5LRw7RF3c2O
5tCWIptu8u8ydIxz9q5zHxxKS+c7q4nkl9V/tVjZx8sneNZB+O79X1teq7LawiYJyLulUMi
OEoiL1YH1SE1U93bUcOWvpAQ5 [email protected]
Select the first characters of the middle blob (after ssh-rsa); this is Base64 and OpenSSL can decode that:
echo "AAAAB3NzaC1yc2EAAAADAQABAAABAQDDo2xko99piegEDgZC" | openssl base64 -d | hd
OpenSSL is picky, it will require that you input no more than 76 characters per line, and the number of characters must be a multiple of 4. The line above will print out this:
00000000 00 00 00 07 73 73 68 2d 72 73 61 00 00 00 03 01 |....ssh-rsa.....|
00000010 00 01 00 00 01 01 00 c3 a3 6c 64 a3 df 69 89 e8 |.........ld..i..|
00000020 04 0e 06 42 |...B|
This reads as such:
00 00 00 07 The length in bytes of the next field
73 73 68 2d 72 73 61 The key type (ASCII encoding of "ssh-rsa")
00 00 00 03 The length in bytes of the public exponent
01 00 01 The public exponent (usually 65537, as here)
00 00 01 01 The length in bytes of the modulus (here, 257)
00 c3 a3... The modulus
So the key has type RSA, and its modulus has length 257 bytes, except that the first byte has value "00", so the real length is 256 bytes (that first byte was added so that the value is considered positive, because the internal encoding rules call for signed integers, the first bit defining the sign). 256 bytes is 2048 bits.
answered Sep 11, 2013 at 17:38
-
Wow, that's complex. But, well done. I'd have thought there was a way to read in the key and output all of that stuff. Commented Sep 11, 2013 at 18:10
-
3For OpenSSL, public keys exist only in certificates or certificate requests, with an ASN.1-based type called
SubjectPublicKeyInfo, different from what SSH does. At the crypto level, a RSA public key is a couple of big integers; how to encode a public key into bytes is out of scope of RSA "stricto sensu" and is up to the protocol which uses it. SSH, X.509, OpenPGP... all have their own encoding conventions. OpenSSL follows the "X.509" religion.– Tom LeekCommented Sep 11, 2013 at 18:33 -
5This answer on Stackoverflow digs into the gory format and binary details. TLDR: the binary blobs used by SSH are not DER (ASN.1), so OpenSSL won't read them directly. If you really wanted to waste 10 minutes you can use
openssl asn1parse -genconfan adapt one of these examples to build a DER format which you can parse with OpenSSL... Commented Sep 11, 2013 at 18:43 -
2
-
4
hdis a specialized keys ofhexdump, a usual Linux tool. On OS X, try:od -t x1– Tom LeekCommented Feb 15, 2014 at 14:02
ssh-keygen -lf /etc/ssh/rsa_key.pub
2048 d1:cb:15:df:5d:44:...
2048 is the keylength
-
1Interesting, can I do it with openssl too, since it is just an rsa public key? Commented Sep 11, 2013 at 17:16
-
For a script that checks all public keys in a file at once see Given keys in ~/.ssh/authorized_keys format, can you determine key strength easily? Commented Aug 11, 2015 at 13:10