To use RSA with Mbed TLS or any other application, you will most likely need an RSA key pair. An RSA key pair is often stored in either a PEM file or a DER file.

Building the RSA key pair generator

Mbed TLS ships with the source code for an RSA key pair generator application, called gen_key. To build the executable for the application, please check out the building Mbed TLS.

After the compilation, the executable is often located in programs/pkey/gen_key.

The gen_key application has the following arguments and options:

usage: gen_key param=<>...

acceptable parameters:
   type=rsa|ec        default: rsa
   rsa_keysize=%d     default: 4096
   ec_curve=%s        see below
   filename=%s        default: keyfile.key
   format=pem|der     default: pem
   use_dev_random=0|1 default: 0

available ec_curve values:
   secp521r1 (default)
   brainpoolP512r1
   secp384r1
   brainpoolP384r1
   secp256r1
   secp256k1
   brainpoolP256r1
   secp224r1
   secp224k1
   secp192r1
   secp192k1

Generating an RSA key pair

To actually generate an RSA key pair, you have to decide which size your RSA key should be. Depending on your need for security, we advise you to use at least 2048 bits, and use 4096 bits or higher if you have very high security requirements.

To use the RSA key pair generator to generate a 4096 bits RSA key and save that key in PEM format in private.key, use:

./gen_key type=rsa rsa_keysize=4096 filename=private.key format=pem

The larger the requested keysize, the longer it will take to generate the key itself. You also need to take into account the performance of the system.

The key generator also outputs the key in human readable format in addition to writing the key to the requested keyfile in the requested format.

The output for a 1024-bit RSA key looks something like this:

  . Seeding the random number generator...
  . Generating the private key ... ok
  . Key information:
N:  8C3A0BE5B196E6CAAEB18B53C654B59FD1F4D1B62CA3C17B4BD094582A4D94BBA9FB3A93E61B4ED0108021A6DFDB2FE89E855030310A7653DC595259501ADC48C4E8C87D78DE750FC44A3069B4F71107C50CDF2E0BD26229297C31D2CBD9C31009736D0D6503ED16D148AF3894770E084BCF56FD6290E1FF28632CB9620B105
E:  010001
D:  1F3728C0D818AC85BFFFBBF1493C403634A9D82942A4AD8481EB0BAAD8B7EE7D1C28D5F0688FF9AA6D91E86BDA7753A1E7EFEB5ADEFFE23966DAE76F045B0660407392B3C07DB8F1200EF70D9CA46DFD3631F455C97F087B8F678DB890AC601FD812C7A435D6826811474BDFACAB2D6AEC9C95809D28AC85DA067B9DA5E005C1
P:  FF96AC7E4E47FC4F153C6C5D3FE7EFC01ECDBBFC6AC861089C675191987281E18E33A731BE939B80C566199E98FDDC9F764AA5283E43FFBA260650B6E9C650D5
Q:  8C73D538DBEA066F00161DCFB69921374ABBD20A96C2693E5072A0890728FE79E881EC532C8918C486C6061987EAED3350F5C6F79E30721C55E3ED72FFF47771
DP: B93FEF6458BECA3C53AB08EC099EC5621DB186786CB931C3790155D8CE82E86AA09405B2036C9F4015536C6C4C7D6BC4548BC3EB483E270337FE49D059DEB8C5
DQ:  61360AEE3FC7CA8C2953256F0EC30DEA671F78513BE773505DFDF87EDF25D07C30213CA094C28F11F6F63862936056AC9DEC7EBA041323E7D8CAD91E2F69D501
QP:  A945E282018B46EFFA11030152A42AD977FAAD439D6C482134A8BC5716F082808904C2001F9D5BBF8B1A6CF98C30FD66BA00EA86F8790552F160929CF10BF8F6

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