Convert pem key to ssh-rsa format
OpensslOpensshOpenssl Problem Overview
I have a certificate in der
format, from it with this command I generate a public key:
openssl x509 -inform der -in ejbcacert.cer -noout -pubkey > pub1key.pub
Which results in this:
-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC7vbqajDw4o6gJy8UtmIbkcpnk
O3Kwc4qsEnSZp/TR+fQi62F79RHWmwKOtFmwteURgLbj7D/WGuNLGOfa/2vse3G2
eHnHl5CB8ruRX9fBl/KgwCVr2JaEuUm66bBQeP5XeBotdR4cvX38uPYivCDdPjJ1
QWPdspTBKcxeFbccDwIDAQAB
-----END PUBLIC KEY-----
How can I obtain a public key like this? Either from certificate or from this public key?
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQC7vbqajDw4o6gJy8UtmIbkcpnkO3Kwc4qsEnSZp/TR+fQi62F79RHWmwKOtFmwteURgLbj7D/WGuNLGOfa/2vse3G2eHnHl5CB8ruRX9fBl/KgwCVr2JaEuUm66bBQeP5XeBotdR4cvX38uPYivCDdPjJ1QWPdspTBKcxeFbccDw==
This was obtained with this command:
ssh-keygen -y -f private_key1.pem > public_key1.pub
Openssl Solutions
Solution 1 - Openssl
No need to compile stuff. You can do the same with ssh-keygen
:
ssh-keygen -f pub1key.pub -i
will read the public key in openssl format from pub1key.pub
and output it in OpenSSH format.
Note: In some cases you will need to specify the input format:
ssh-keygen -f pub1key.pub -i -mPKCS8
From the ssh-keygen docs (From man ssh-keygen):
> -m key_format Specify a key format for the -i (import) or -e (export) conversion options. The supported key formats are: “RFC4716” (RFC 4716/SSH2 public or private key), “PKCS8” (PEM PKCS8 public key) or “PEM” (PEM public key). The default conversion format is “RFC4716”.
Solution 2 - Openssl
No need for scripts or other 'tricks': openssl
and ssh-keygen
are enough. I'm assuming no password for the keys (which is bad).
Generate an RSA pair
All the following methods give an RSA key pair in the same format
-
With openssl ([man genrsa][4])
openssl genrsa -out dummy-genrsa.pem 2048
In OpenSSL v1.0.1 genrsa
[is superseded][1] by genpkey
so this is the new way to do it ([man genpkey][3]):
openssl genpkey -algorithm RSA -out dummy-genpkey.pem -pkeyopt rsa_keygen_bits:2048
2. With ssh-keygen
ssh-keygen -t rsa -b 2048 -f dummy-ssh-keygen.pem -N '' -C "Test Key"
Converting DER to PEM
If you have an RSA key pair in DER format, you may want to convert it to PEM to allow the format conversion below:
Generation:
openssl genpkey -algorithm RSA -out genpkey-dummy.cer -outform DER -pkeyopt rsa_keygen_bits:2048
Conversion:
openssl rsa -inform DER -outform PEM -in genpkey-dummy.cer -out dummy-der2pem.pem
Extract the public key from the PEM formatted RSA pair
-
in PEM format:
openssl rsa -in dummy-xxx.pem -pubout
-
in OpenSSH v2 format [see][2]:
ssh-keygen -y -f dummy-xxx.pem
Notes
OS and software version:
[user@test1 ~]# cat /etc/redhat-release ; uname -a ; openssl version
CentOS release 6.5 (Final)
Linux test1.example.local 2.6.32-431.el6.x86_64 #1 SMP Fri Nov 22 03:15:09 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
OpenSSL 1.0.1e-fips 11 Feb 2013
References:
[1]: https://www.openssl.org/docs/apps/openssl.html "man openssl" [2]: http://www.sysmic.org/dotclear/index.php?post/2010/03/24/Convert-keys-betweens-GnuPG%2C-OpenSsh-and-OpenSSL [3]: https://www.openssl.org/docs/apps/genpkey.html "man genpkey" [4]: https://www.openssl.org/docs/apps/genrsa.html "man genrsa" [5]: http://www.openssl.org/docs/HOWTO/keys.txt
Solution 3 - Openssl
To answer my own question, after posting on openssl mailing list got this:
Here is C code to convert from an OpenSSL public key to an OpenSSH public key. You can grab the code from this link and compile it yourself:
static unsigned char pSshHeader[11] = { 0x00, 0x00, 0x00, 0x07, 0x73, 0x73, 0x68, 0x2D, 0x72, 0x73, 0x61};
static int SshEncodeBuffer(unsigned char *pEncoding, int bufferLen, unsigned char* pBuffer)
{
int adjustedLen = bufferLen, index;
if (*pBuffer & 0x80)
{
adjustedLen++;
pEncoding[4] = 0;
index = 5;
}
else
{
index = 4;
}
pEncoding[0] = (unsigned char) (adjustedLen >> 24);
pEncoding[1] = (unsigned char) (adjustedLen >> 16);
pEncoding[2] = (unsigned char) (adjustedLen >> 8);
pEncoding[3] = (unsigned char) (adjustedLen );
memcpy(&pEncoding[index], pBuffer, bufferLen);
return index + bufferLen;
}
int main(int argc, char** argv)
{
int iRet = 0;
int nLen = 0, eLen = 0;
int encodingLength = 0;
int index = 0;
unsigned char *nBytes = NULL, *eBytes = NULL;
unsigned char* pEncoding = NULL;
FILE* pFile = NULL;
EVP_PKEY *pPubKey = NULL;
RSA* pRsa = NULL;
BIO *bio, *b64;
ERR_load_crypto_strings();
OpenSSL_add_all_algorithms();
if (argc != 3)
{
printf("usage: %s public_key_file_name ssh_key_description\n", argv[0]);
iRet = 1;
goto error;
}
pFile = fopen(argv[1], "rt");
if (!pFile)
{
printf("Failed to open the given file\n");
iRet = 2;
goto error;
}
pPubKey = PEM_read_PUBKEY(pFile, NULL, NULL, NULL);
if (!pPubKey)
{
printf("Unable to decode public key from the given file: %s\n", ERR_error_string(ERR_get_error(), NULL));
iRet = 3;
goto error;
}
if (EVP_PKEY_type(pPubKey->type) != EVP_PKEY_RSA)
{
printf("Only RSA public keys are currently supported\n");
iRet = 4;
goto error;
}
pRsa = EVP_PKEY_get1_RSA(pPubKey);
if (!pRsa)
{
printf("Failed to get RSA public key : %s\n", ERR_error_string(ERR_get_error(), NULL));
iRet = 5;
goto error;
}
// reading the modulus
nLen = BN_num_bytes(pRsa->n);
nBytes = (unsigned char*) malloc(nLen);
BN_bn2bin(pRsa->n, nBytes);
// reading the public exponent
eLen = BN_num_bytes(pRsa->e);
eBytes = (unsigned char*) malloc(eLen);
BN_bn2bin(pRsa->e, eBytes);
encodingLength = 11 + 4 + eLen + 4 + nLen;
// correct depending on the MSB of e and N
if (eBytes[0] & 0x80)
encodingLength++;
if (nBytes[0] & 0x80)
encodingLength++;
pEncoding = (unsigned char*) malloc(encodingLength);
memcpy(pEncoding, pSshHeader, 11);
index = SshEncodeBuffer(&pEncoding[11], eLen, eBytes);
index = SshEncodeBuffer(&pEncoding[11 + index], nLen, nBytes);
b64 = BIO_new(BIO_f_base64());
BIO_set_flags(b64, BIO_FLAGS_BASE64_NO_NL);
bio = BIO_new_fp(stdout, BIO_NOCLOSE);
BIO_printf(bio, "ssh-rsa ");
bio = BIO_push(b64, bio);
BIO_write(bio, pEncoding, encodingLength);
BIO_flush(bio);
bio = BIO_pop(b64);
BIO_printf(bio, " %s\n", argv[2]);
BIO_flush(bio);
BIO_free_all(bio);
BIO_free(b64);
error:
if (pFile)
fclose(pFile);
if (pRsa)
RSA_free(pRsa);
if (pPubKey)
EVP_PKEY_free(pPubKey);
if (nBytes)
free(nBytes);
if (eBytes)
free(eBytes);
if (pEncoding)
free(pEncoding);
EVP_cleanup();
ERR_free_strings();
return iRet;
}
Solution 4 - Openssl
ssh-keygen -i -m PKCS8 -f public-key.pem
Solution 5 - Openssl
ssh-keygen -f private.pem -y > public.pub
Solution 6 - Openssl
The following script would obtain the ci.jenkins-ci.org public key certificate in base64-encoded DER format and convert it to an OpenSSH public key file. This code assumes that a 2048-bit RSA key is used and draws a lot from this Ian Boyd's answer. I've explained a bit more how it works in comments to this article in Jenkins wiki.
echo -n "ssh-rsa " > jenkins.pub
curl -sfI https://ci.jenkins-ci.org/ | grep -i X-Instance-Identity | tr -d \\r | cut -d\ -f2 | base64 -d | dd bs=1 skip=32 count=257 status=none | xxd -p -c257 | sed s/^/00000007\ 7373682d727361\ 00000003\ 010001\ 00000101\ / | xxd -p -r | base64 -w0 >> jenkins.pub
echo >> jenkins.pub
Solution 7 - Openssl
Solution 8 - Openssl
FWIW, this BASH script will take a PEM- or DER-format X.509 certificate or OpenSSL public key file (also PEM format) as the first argument and disgorge an OpenSSH RSA public key. This expands upon @mkalkov's answer above. Requirements are cat
, grep
, tr
, dd
, xxd
, sed
, xargs
, file
, uuidgen
, base64
, openssl
(1.0+), and of course bash
. All except openssl
(contains base64
) are pretty much guaranteed to be part of the base install on any modern Linux system, except maybe xxd
(which Fedora shows in the vim-common
package). If anyone wants to clean it up and make it nicer, caveat lector.
#!/bin/bash
#
# Extract a valid SSH format public key from an X509 public certificate.
#
# Variables:
pubFile=$1
fileType="no"
pkEightTypeFile="$pubFile"
tmpFile="/tmp/`uuidgen`-pkEightTypeFile.pk8"
# See if a file was passed:
[ ! -f "$pubFile" ] && echo "Error, bad or no input file $pubFile." && exit 1
# If it is a PEM format X.509 public cert, set $fileType appropriately:
pemCertType="X$(file $pubFile | grep 'PEM certificate')"
[ "$pemCertType" != "X" ] && fileType="PEM"
# If it is an OpenSSL PEM-format PKCS#8-style public key, set $fileType appropriately:
pkEightType="X$(grep -e '-BEGIN PUBLIC KEY-' $pubFile)"
[ "$pkEightType" != "X" ] && fileType="PKCS"
# If this is a file we can't recognise, try to decode a (binary) DER-format X.509 cert:
if [ "$fileType" = "no" ]; then
openssl x509 -in $pubFile -inform DER -noout
derResult=$(echo $?)
[ "$derResult" = "0" ] && fileType="DER"
fi
# Exit if not detected as a file we can use:
[ "$fileType" = "no" ] && echo "Error, input file not of type X.509 public certificate or OpenSSL PKCS#8-style public key (not encrypted)." && exit 1
# Convert the X.509 public cert to an OpenSSL PEM-format PKCS#8-style public key:
if [ "$fileType" = "PEM" -o "$fileType" = "DER" ]; then
openssl x509 -in $pubFile -inform $fileType -noout -pubkey > $tmpFile
pkEightTypeFile="$tmpFile"
fi
# Build the string:
# Front matter:
frontString="$(echo -en 'ssh-rsa ')"
# Encoded modulus and exponent, with appropriate pointers:
encodedModulus="$(cat $pkEightTypeFile | grep -v -e "----" | tr -d '\n' | base64 -d | dd bs=1 skip=32 count=257 status=none | xxd -p -c257 | sed s/^/00000007\ 7373682d727361\ 00000003\ 010001\ 00000101\ / | xxd -p -r | base64 -w0 )"
# Add a comment string based on the filename, just to be nice:
commentString=" $(echo $pubFile | xargs basename | sed -e 's/\.crt\|\.cer\|\.pem\|\.pk8\|\.der//')"
# Give the user a string:
echo $frontString $encodedModulus $commentString
# cleanup:
rm -f $tmpFile