SSL_alert_type_string() returns a one letter string indicating the
type of the alert specified by value.
SSL_alert_type_string_long() returns a string indicating the type of
the alert specified by value.
SSL_alert_desc_string() returns a two letter string as a short form
describing the reason of the alert specified by value.
SSL_alert_desc_string_long() returns a string describing the reason
of the alert specified by value.
NOTES
When one side of an SSL/TLS communication wants to inform the peer
about a special situation, it sends an alert. The alert is sent as a
special message and does not influence the normal data stream (unless
its contents results in the communication being canceled).
A warning alert is sent, when a non-fatal error condition occurs. The
"close notify" alert is sent as a warning alert. Other examples for
non-fatal errors are certificate errors ("certificate expired",
"unsupported certificate"), for which a warning alert may be sent.
(The sending party may however decide to send a fatal error.) The
receiving side may cancel the connection on reception of a warning
alert on it discretion.
Several alert messages must be sent as fatal alert messages as
specified by the TLS RFC. A fatal alert always leads to a connection
abort.
RETURN VALUES
The following strings can occur for SSL_alert_type_string() or
SSL_alert_type_string_long():
"W"/"warning"
"F"/"fatal"
"U"/"unknown"
This indicates that no support is available for this alert
type. Probably value does
not contain a correct alert message.
The following strings can occur for SSL_alert_desc_string() or
SSL_alert_desc_string_long():
"CN"/"close notify"
The connection shall be closed. This is a warning alert.
"UM"/"unexpected message"
An inappropriate message was received. This alert is always
fatal and should never be observed in communication between
proper implementations.
"BM"/"bad record mac"
This alert is returned if a record is received with an
incorrect MAC. This message is always fatal.
"DF"/"decompression failure"
The decompression function received improper input (e.g. data
that would expand to excessive length). This message is
always fatal.
"HF"/"handshake failure"
Reception of a handshake_failure alert message indicates that
the sender was unable to negotiate an acceptable set of
security parameters given the options available. This is a
fatal error.
"NC"/"no certificate"
A client, that was asked to send a certificate, does not send
a certificate (SSLv3 only).
"BC"/"bad certificate"
A certificate was corrupt, contained signatures that did not
verify correctly, etc
"UC"/"unsupported certificate"
A certificate was of an unsupported type.
"CR"/"certificate revoked"
A certificate was revoked by its signer.
"CE"/"certificate expired"
A certificate has expired or is not currently valid.
"CU"/"certificate unknown"
Some other (unspecified) issue arose in processing the
certificate, rendering it unacceptable.
"IP"/"illegal parameter"
A field in the handshake was out of range or inconsistent
with other fields. This is always fatal.
"DC"/"decryption failed"
A TLSCiphertext decrypted in an invalid way: either it wasn't
an even multiple of the block length or its padding values,
when checked, weren't correct. This message is always fatal.
"RO"/"record overflow"
A TLSCiphertext record was received which had a length more
than 2^14+2048 bytes, or a record decrypted to a
TLSCompressed record with more than 2^14+1024 bytes. This
message is always fatal.
"CA"/"unknown CA"
A valid certificate chain or partial chain was received, but
the certificate was not accepted because the CA certificate
could not be located or couldn't be matched with a known,
trusted CA. This message is always fatal.
"AD"/"access denied"
A valid certificate was received, but when access control was
applied, the sender decided not to proceed with negotiation.
This message is always fatal.
"DE"/"decode error"
A message could not be decoded because some field was out of
the specified range or the length of the message was
incorrect. This message is always fatal.
"CY"/"decrypt error"
A handshake cryptographic operation failed, including being
unable to correctly verify a signature, decrypt a key
exchange, or validate a finished message.
"ER"/"export restriction"
A negotiation not in compliance with export restrictions was
detected; for example, attempting to transfer a 1024 bit
ephemeral RSA key for the RSA_EXPORT handshake method. This
message is always fatal.
"PV"/"protocol version"
The protocol version the client has attempted to negotiate is
recognized, but not supported. (For example, old protocol
versions might be avoided for security reasons). This message
is always fatal.
"IS"/"insufficient security"
Returned instead of handshake_failure when a negotiation has
failed specifically because the server requires ciphers more
secure than those supported by the client. This message is
always fatal.
"IE"/"internal error"
An internal error unrelated to the peer or the correctness of
the protocol makes it impossible to continue (such as a
memory allocation failure). This message is always fatal.
"US"/"user canceled"
This handshake is being canceled for some reason unrelated to
a protocol failure. If the user cancels an operation after
the handshake is complete, just closing the connection by
sending a close_notify is more appropriate. This alert should
be followed by a close_notify. This message is generally a
warning.
"NR"/"no renegotiation"
Sent by the client in response to a hello request or by the
server in response to a client hello after initial
handshaking. Either of these would normally lead to
renegotiation; when that is not appropriate, the recipient
should respond with this alert; at that point, the original
requester can decide whether to proceed with the connection.
One case where this would be appropriate would be where a
server has spawned a process to satisfy a request; the
process might receive security parameters (key length,
authentication, etc.) at startup and it might be difficult to
communicate changes to these parameters after that point.
This message is always a warning.
"UK"/"unknown"
This indicates that no description is available for this
alert type. Probably value
does not contain a correct alert message.
