Crypto-Gram is a famous free monthly newsletter from security expert Bruce Schneier. The original it’s available at Bruce Schneier’s website; this is the same content in MOBI and EPUB format, suitable for e-book readers.

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• Crypto-Gram, April 2021 issue, in MOBI format (for Amazon Kindle)
• Crypto-Gram, April 2021 issue, in EPUB format (for other e-book readers)