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 7.2 Used S-expressions
 ======================
 
 Libgcrypt's API for asymmetric cryptography is based on data structures
 called S-expressions (see XXXX) and does not work with contexts as most
 of the other building blocks of Libgcrypt do.
 
    The following information are stored in S-expressions:
 
 keys
 
 plain text data
 
 encrypted data
 
 signatures
 
 To describe how Libgcrypt expect keys, we use some examples. Note that
 words in uppercase indicate parameters whereas lowercase words are
 literals.
 
      (private-key
        (dsa
          (p P-MPI)
          (q Q-MPI)
          (g G-MPI)
          (y Y-MPI)
          (x X-MPI)))
 
 This specifies a DSA private key with the following parameters:
 
 P-MPI
      DSA prime p.
 
 Q-MPI
      DSA group order q (which is a prime divisor of p-1).
 
 G-MPI
      DSA group generator g.
 
 Y-MPI
      DSA public key value y = g^x \bmod p.
 
 X-MPI
      DSA secret exponent x.
 
    All the MPI values are  expected to be in `GCRYMPI_FMT_USG' format.
 The public key is similar with "private-key" replaced by "public-key"
 and no X-MPI.
 
    An easy way to create such an S-expressions is by using
 `gcry_sexp_build' which allows to pass a string with printf-like
 escapes to insert MPI values.
 
 Here is an example for an RSA key:
 
      (private-key
        (rsa
          (n N-MPI)
          (e E-MPI)
          (d D-MPI)
          (p P-MPI)
          (q Q-MPI)
          (u U-MPI)
 
 with
 
 N-MPI
      RSA public modulus n.
 
 E-MPI
      RSA public exponent e.
 
 D-MPI
      RSA secret exponent d = e^-1 \bmod (p-1)(q-1).
 
 P-MPI
      RSA secret prime p.
 
 Q-MPI
      RSA secret prime q with q > p.
 
 U-MPI
      multiplicative inverse u = p^-1 \bmod q.
 

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