libaaruformat/docs/spec/structs/checksum.adoc

=== Checksum Block (`CKSM`)

This block stores an array of checksums corresponding to the user data embedded in the image.
For media formats such as CompactDisc, the checksum is calculated over the complete sector—comprising the prefix, user data, and suffix—totaling 2352 bytes.

If the image is modified, the checksum block is considered outdated and should be either removed or excluded from the most recent index to ensure integrity.

==== Structure Definition

[source,c]
#define CHECKSUM_MAGIC 0x4D534B43
/**
 *     Checksum block, contains a checksum of all user data sectors (except for optical discs that is 2352 uint8_ts raw
 *     sector if available
 *  */
typedef struct ChecksumHeader
{
    /**Identifier, <see cref="BlockType.ChecksumBlock" /> */
    uint32_t identifier;
    /**Length in uint8_ts of the block */
    uint32_t length;
    /**How many checksums follow */
    uint8_t  entries;
} ChecksumHeader;

==== Field Descriptions

[cols="2,2,2,6",options="header"]
|===
|Type
|Size
|Name
|Description

|uint32_t
|4 bytes
|identifier
|The tracks block identifier, always `CKSM`

|uint32_t
|4 bytes
|length
|The length in bytes of the data following this header.

|uint8_t
|1 byte
|entries
|The number of entries following this header
|===

==== Checksum entries

[source,c]
/**Checksum entry, followed by checksum data itself */
typedef struct ChecksumEntry
{
    /**Checksum algorithm */
    uint8_t  type;
    /**Length in uint8_ts of checksum that follows this structure */
    uint32_t length;
} ChecksumEntry;

==== Field Descriptions

[cols="2,2,2,6",options="header"]
|===
|Type
|Size
|Name
|Description

|uint8_t
|1 byte
|type
|Checksum algorithm.

|uint32_t
|4 bytes
|length
|Size in bytes of the checksum that immediately follows this entry.
|===

==== Checksum algorithms

[cols="2,2,6",options="header"]
|===
|Type
|Value
|Description

|Invalid
|0
|Invalid checksum entry, skip.

|Md5
|1
|MD5

|Sha1
|2
|SHA1

|Sha256
|3
|SHA-256

|SpamSum
|4
|SpamSum
|===