bucket-A-inline-policy.json

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "ReadBucketA",
            "Effect": "Allow",
            "Action": [
                "s3:ListBucket",
                "s3:GetObject"
            ],
            "Resource": [
                "arn:aws:s3:::bucket-A",
                "arn:aws:s3:::bucket-A/*"
            ]
        }
    ]
}

bucket-B-inline policy-encrypted.json

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "grantKMSKey",
            "Effect": "Allow",
            "Action": [
                "kms:GenerateDataKey",
                "kms:Encrypt",
                "kms:DescribeKey",
                "kms:Decrypt"
            ],
            "Resource": "arn:aws:kms:eu-west-3:ACCOUNT_B:key/XXXX"
        },
        {
            "Sid": "ReadBucketB",
            "Effect": "Allow",
            "Action": [
                "s3:ListBucket",
                "s3:GetObject"
            ],
            "Resource": "arn:aws:s3:::bucket-B"
        },
        {
            "Sid": "writeBucketB",
            "Effect": "Allow",
            "Action": [
                "s3:RestoreObject",
                "s3:PutObject",
                "s3:DeleteObject"
            ],
            "Resource": "arn:aws:s3:::bucket-B"
        }
    ]
}