+=head1 GCM and OCB Modes
+
+For GCM and OCB mode ciphers the behaviour of the EVP interface is subtly
+altered and several additional ctrl operations are supported.
+
+To specify any additional authenticated data (AAD) a call to EVP_CipherUpdate(),
+EVP_EncryptUpdate() or EVP_DecryptUpdate() should be made with the output
+parameter B<out> set to B<NULL>.
+
+When decrypting the return value of EVP_DecryptFinal() or EVP_CipherFinal()
+indicates if the operation was successful. If it does not indicate success
+the authentication operation has failed and any output data B<MUST NOT>
+be used as it is corrupted.
+
+The following ctrls are supported in both GCM and OCB modes:
+
+ EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_SET_IVLEN, ivlen, NULL);
+
+Sets the IV length: this call can only be made before specifying an IV. If
+not called a default IV length is used. For GCM AES and OCB AES the default is
+12 (i.e. 96 bits). For OCB mode the maximum is 15.
+
+ EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_GET_TAG, taglen, tag);
+
+Writes B<taglen> bytes of the tag value to the buffer indicated by B<tag>.
+This call can only be made when encrypting data and B<after> all data has been
+processed (e.g. after an EVP_EncryptFinal() call). For OCB mode the taglen must
+either be 16 or the value previously set via EVP_CTRL_OCB_SET_TAGLEN.
+
+ EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_SET_TAG, taglen, tag);
+
+Sets the expected tag to B<taglen> bytes from B<tag>. This call is only legal
+when decrypting data and must be made B<before> any data is processed (e.g.
+before any EVP_DecryptUpdate() call). For OCB mode the taglen must
+either be 16 or the value previously set via EVP_CTRL_AEAD_SET_TAG.
+
+In OCB mode calling this with B<tag> set to NULL sets the tag length. The tag
+length can only be set before specifying an IV. If not called a default tag
+length is used. For OCB AES the default is 16 (i.e. 128 bits). This is also the
+maximum tag length for OCB.
+
+See L</EXAMPLES> below for an example of the use of GCM mode.
+
+=head1 CCM Mode
+
+The behaviour of CCM mode ciphers is similar to GCM mode but with a few
+additional requirements and different ctrl values.
+
+Like GCM and OCB modes any additional authenticated data (AAD) is passed by calling
+EVP_CipherUpdate(), EVP_EncryptUpdate() or EVP_DecryptUpdate() with the output
+parameter B<out> set to B<NULL>. Additionally the total plaintext or ciphertext
+length B<MUST> be passed to EVP_CipherUpdate(), EVP_EncryptUpdate() or
+EVP_DecryptUpdate() with the output and input parameters (B<in> and B<out>)
+set to B<NULL> and the length passed in the B<inl> parameter.
+
+The following ctrls are supported in CCM mode:
+
+ EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_SET_TAG, taglen, tag);
+
+This call is made to set the expected B<CCM> tag value when decrypting or
+the length of the tag (with the B<tag> parameter set to NULL) when encrypting.
+The tag length is often referred to as B<M>. If not set a default value is
+used (12 for AES).
+
+ EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_CCM_SET_L, ivlen, NULL);
+
+Sets the CCM B<L> value. If not set a default is used (8 for AES).
+
+ EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_SET_IVLEN, ivlen, NULL);
+
+Sets the CCM nonce (IV) length: this call can only be made before specifying
+an nonce value. The nonce length is given by B<15 - L> so it is 7 by default
+for AES.
+