Convert pem key to ssh-rsa format

Question

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  gt  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  gt  public key1 pub

User · Answer

ssh-keygen -i -m PKCS8 -f public-key pem

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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  amp  0x80             adjustedLen          pEncoding 4    0        index   5          else            index   4          pEncoding 0     unsigned char   adjustedLen  gt  gt  24      pEncoding 1     unsigned char   adjustedLen  gt  gt  16      pEncoding 2     unsigned char   adjustedLen  gt  gt   8      pEncoding 3     unsigned char   adjustedLen            memcpy  amp 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- gt 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- gt n      nBytes    unsigned char   malloc nLen      BN bn2bin pRsa- gt n  nBytes          reading the public exponent    eLen   BN num bytes pRsa- gt e      eBytes    unsigned char   malloc eLen      BN bn2bin pRsa- gt e  eBytes       encodingLength   11   4   eLen   4   nLen        correct depending on the MSB of e and N    if  eBytes 0   amp  0x80        encodingLength       if  nBytes 0   amp  0x80        encodingLength        pEncoding    unsigned char   malloc encodingLength      memcpy pEncoding  pSshHeader  11       index   SshEncodeBuffer  amp pEncoding 11   eLen  eBytes      index   SshEncodeBuffer  amp 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

User · Answer

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   openssl genrsa -out dummy-genrsa pem 2048   In OpenSSL v1 0 1 genrsa is superseded by genpkey so this is the new way to do it  man genpkey    openssl genpkey -algorithm RSA -out dummy-genpkey pem -pkeyopt rsa keygen bits 2048  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   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    Sysmic org Convert keys betweens GnuPG  OpenSsh and OpenSSL

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ssh-keygen -f private pem -y  gt  public pub

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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     amp  amp  echo  Error  bad or no input file  pubFile    amp  amp  exit 1    If it is a PEM format X 509 public cert  set  fileType appropriately  pemCertType  X  file  pubFile   grep  PEM certificate        pemCertType      X     amp  amp  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     amp  amp  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     amp  amp  fileType  DER  fi    Exit if not detected as a file we can use      fileType     no     amp  amp  echo  Error  input file not of type X 509 public certificate or OpenSSL PKCS 8-style public key  not encrypted     amp  amp  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  gt   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

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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  quot ssh-rsa  quot   gt  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  gt  gt  jenkins pub echo  gt  gt  jenkins pub

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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

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I did with  ssh-keygen -i -f  sshkeysfile  gt  gt  authorized keys  Credit goes here

[openssl] Convert pem key to ssh-rsa format

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