Convert between SBOM formats (#964)

* add convert command

Signed-off-by: Jonas Galvão Xavier <jonas.agx@gmail.com>

* mvp

Signed-off-by: Jonas Galvão Xavier <jonasx@anchore.com>

* fix hanging bug

Signed-off-by: Jonas Galvão Xavier <jonasx@anchore.com>

* validate SBOM formats for conversion

Signed-off-by: Jonas Galvão Xavier <jonasx@anchore.com>

* move convert cmd to new structure

Signed-off-by: Jonas Galvão Xavier <jonasx@anchore.com>

* remove bin

Signed-off-by: Jonas Galvão Xavier <jonasx@anchore.com>

* drop event loop from convert cmd

extract SBOM type from document namespace

Signed-off-by: Jonas Galvão Xavier <jonasx@anchore.com>

* validate SPDX in tests

Signed-off-by: Jonas Galvão Xavier <jonasx@anchore.com>

* documenting convert cmd

Signed-off-by: Jonas Galvão Xavier <jonasx@anchore.com>

* support output format=file.json notation

Signed-off-by: Jonas Galvão Xavier <jonasx@anchore.com>

* test convertible formats

Signed-off-by: Jonas Galvão Xavier <jonasx@anchore.com>

* fix typo

Signed-off-by: Jonas Galvão Xavier <jonasx@anchore.com>

* clean up

Signed-off-by: Jonas Galvão Xavier <jonasx@anchore.com>

* more clean up and docs

Signed-off-by: Jonas Galvão Xavier <jonasx@anchore.com>

* feedback changes

Signed-off-by: Jonas Galvão Xavier <jonasx@anchore.com>

* nit

Signed-off-by: Jonas Xavier <jonasx@anchore.com>

* feedback changes

Signed-off-by: Jonas Xavier <jonasx@anchore.com>

* re-use more code

Signed-off-by: Jonas Xavier <jonasx@anchore.com>

* undo encode-decode cycle test

Signed-off-by: Jonas Xavier <jonasx@anchore.com>

* remove unnecessary test constraint

Signed-off-by: Jonas Xavier <jonasx@anchore.com>

* fix readme

Signed-off-by: Jonas Xavier <jonasx@anchore.com>

* try verbose

Signed-off-by: Jonas Xavier <jonasx@anchore.com>

* cleaner README and no table conversion

Signed-off-by: Jonas Xavier <jonasx@anchore.com>

* simpler conversion

Signed-off-by: Jonas Xavier <jonasx@anchore.com>

* feedback changes and cleanup

Signed-off-by: Jonas Xavier <jonasx@anchore.com>

* nit space fix

Signed-off-by: Jonas Xavier <jonasx@anchore.com>

* use defer

Signed-off-by: Jonas Xavier <jonasx@anchore.com>

* feedback changes

Signed-off-by: Jonas Xavier <jonasx@anchore.com>

Co-authored-by: Keith Zantow <kzantow@gmail.com>

README.md

@@ -25,6 +25,7 @@ A CLI tool and Go library for generating a Software Bill of Materials (SBOM) fro
 - Linux distribution identification
 - Works seamlessly with [Grype](https://github.com/anchore/grype) (a fast, modern vulnerability scanner)
 - Able to create signed SBOM attestations using the [in-toto specification](https://github.com/in-toto/attestation/blob/main/spec/README.md)
+- Convert between SBOM formats, such as CycloneDX, SPDX, and Syft's own format.
 
 ### Supported Ecosystems
 
@@ -93,6 +94,29 @@ The above output includes only software that is visible in the container (i.e.,
 syft <image> --scope all-layers
 ```
 
+#### Format conversion (experimental)
+
+The ability to convert existing SBOMs means you can create SBOMs in different formats quickly, without the need to regenerate the SBOM from scratch, which may take significantly more time.
+
+```
+syft convert <ORIGINAL-SBOM-FILE> -o <NEW-SBOM-FORMAT>[=<NEW-SBOM-FILE>]
+```
+
+This feature is experimental and data might be lost when converting formats. Packages are the main SBOM component easily transferable across formats, whereas files and relationships, as well as other information Syft doesn't support, are more likely to be lost. 
+
+We support formats with wide community usage AND good encode/decode support by Syft. The supported formats are:
+- Syft JSON
+- SPDX 2.2 JSON
+- SPDX 2.2 tag-value
+- CycloneDX 1.4 JSON
+- CycloneDX 1.4 XML
+
+Conversion example:
+```sh
+syft alpine:latest -o syft-json=sbom.syft.json # generate a syft SBOM
+syft convert sbom.syft.json -o cyclonedx-json=sbom.cdx.json  # convert it to CycloneDX
+```
+
 #### SBOM attestation
 
 ### Keyless support